[unreadable] We propose to develop a new type of solid-state photodetector, namely the Planar Avalanche Photodiode (PAPD), to replace photomultiplier tubes (PMT) in positron emission tomography (PET) detector modules for breast imaging. Solid-state devices offer significant improvements in performance and cost compared to the PMTs, and also can be operated in magnetic fields, which opens up the additional possibility of dual-modality PET/MRI types of imaging instruments. This new avalanche photodiode (APD) design is expected to be much more reliable, robust and lower cost to produce than the current commercially available APDs. In addition, and perhaps more importantly, the new design allows for easy implementation of large area arrays, unlike other current APDs that are being developed for PMT replacements. [unreadable] [unreadable] The new PAPD design will be based on experience we have accumulated over several years in the design and fabrication of standard silicon p-i-n photodiodes, beveled-edge APDs, and silicon drift photodiodes. In Phase I, we will develop the prototype PAPD detector, which will include computer simulations of the design, design of photomasks and process sequence, fabrication and testing of the PAPDs. The testing of the PAPDs will include dark current, response to light from light emitting diodes, quantum efficiency, gain measurements, and response to gamma radiation when coupled to scintillators. Phase I results will show the feasibility of the new design as applied towards larger area arrays for PET detector modules. [unreadable] [unreadable] In Phase II, we will optimize the PAPD design based on the Phase I results. 8x8 arrays of the PAPDs will be fabricated and tested. Two complete detector modules, based on 8x8 arrays of PAPDs, coupled to 8x8 arrays of LSO scintillators, will be assembled with full readout electronics and software. The detector modules will be characterized and tested with respect to energy resolution and timing in response to gammaradiation. Full evaluation as prototype PET detector modules will be performed in collaboration with our colleagues at UCLA in their PET test setup. [unreadable] [unreadable]